This invention relates to a process for the production of germanes such as monogermane, digermane and trigermane and particularly, to a process for the production of germanes by reducing germanium halides with use of a mixture of alkylaluminum hydride and trialkylaluminum.
Germanes or germanium hydrides are useful as a gas for the production of semiconductors and particularly, monogermane is valuable as a gas for doping.
For the production of germanes, there are known a process by reacting a magnesium-germanium alloy with an acid (J. of American Chemical Society, Vol. 46, p. 657 (1924)) and a process by reducing germanium oxide with sodium borohydride in an aqueous solution (J. of Inorg. Nuclear Chemistry, Vol. 4, pp 22-23 (1957)). However, in the former process, higher order germanes are by produced and a yield ratio of lower order germanes particularly, monogermane to the starting germanium is low (20% or less). Also, in the latter process, expensive sodium borohydride is used as a reducing agent as well as needing bromic acid as a reaction assistant and the selection of reaction conditions is difficult and further, the yield of end products is not satisfactory.
Furthermore, there is reported a process for the production of monogermane by reducing germanium tetrachloride with a reducing agent, for example, sodium borohydride and lithium aluminumhydride. This process, however, has disadvantages that the reducing agent is not only expensive, but the yield is not satisfactory (for example, J. of Chemical Society, pp 1984-1988 (1959)).
Also, there is known a process using sodium hydride relatively inexpensive as compared with the above-mentioned reducing agents. This process, however, needs using an expensive solvent such as diethyleneglycoldimethylether and adding a hydride soluble in said solvent such as sodium borohydride as a reduction assistant to the reaction system and accordingly, it is not convenient for industrial practice (Belgian Patent No. 890,356).
Furthermore, there is known a process for producing monogermane by reacting germanium tetrachloride with diisobutylaluminum hydride which is available in a commercial scale (Chemical Abstracts, Vol. 76, 20761W). According to this process, the yield is relatively good, but the resulting germanes have impurities such as other germanium hydrides than germanes and hydrocarbon compounds and accordingly, a large-scaled purification apparatus is necessary for use as the gas for semiconductors and thus, it is substantially impossible to carry out this process in a commercial scale.
On the other hand, as to alkylaluminum hydride it is difficult to industrially obtain it in pure form, but the alkylaluminum hydride is available easily and inexpensively in a commercial scale in the form of mixture with trialkylaluminum. Therefore, it is very valuable industrially if germanes can be effectively obtained by carrying out the reduction reaction with use of such a mixture of alkylaluminum hydride with trialkylaluminum.
The inventors have studied the reduction reaction of germanium halide using the above mixture and as a result, found that germanes obtained are very low in yield and monoethyl germanium (GeH.sub.3 C.sub.2 H.sub.5) and monochlorogermanium (GeH.sub.3 Cl) are by-produced in large amount, probably by insufficient reduction of the germanium halide and also, ethane is formed in large amount, perhaps by side reactions.